EARTH SCIENCES DEPARTMENT SEMINAR
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Unique PGE-Cu-Ni Noril’sk deposits: geology and origin
Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
Tuesday 14th January - 4.00 pm
EARTH SCIENCES SEMINAR ROOM
Thanks to their uniqueness in the extensive class of magmatic Pt-Cu-Ni deposits (their setting in the flood-basalt province, young age, and the vast thickness of the ores related to the relatively thin intrusive bodies), the Norilsk ore-bearing massifs continue to attract keen interest of researchers during more than five decades. The paramount impact of the discovery of the Talnakh deposits on the world's economy still puts forth the problems of the genesis of such ores. Solving these problems will facilitate in optimizing exploration for such unique ores.
Although the Norilsk deposits have been studied for a long time, several issues of their genesis remain obscure until nowadays. A principally important problem is the mechanisms that concentrated metals in the uniquely large deposits. Several hypotheses were suggested to explain this phenomenon. Some researchers explained the unusual structure of the deposits by their origin from unusual ore-bearing magmas, others argued that the deposits were produced by tholeiitic melts during their long-lasting ascent to the surface. Practically all of the genetic models attach much importance to the assimilation of rocks, first of all, anhydrite, which provided sulfur for the system.
Our study of geological relations between basalts and intrusions in the Norilsk Complex and on their major- and trace-element compositions (6 - 7 wt % MgO in the volcanic rocks and 10 - 12 in the intrusions, relatively low Ti concentrations and La/Yb ratios in rocks of the Norilsk Complex) and isotopic composition (first of all, sulfur isotopic composition δ34S from +1 - +5 to +18‰ for the basalts and intrusions, respectively), the conclusion was drawn that the ore-bearing intrusions have no comagmatic volcanic rocks and were produced by a separate magmatic pulse in post-Nadezhdinsky time. There is much less evidence that the magmas of the Norilsk Complex were emplaced in post-Morongovskoe time and, perhaps, even after the whole volcanic pile was formed (Malich et al., 2010; Ivanov, 2011).
We were the first to widely apply a new approach to estimating the composition of the parental melt of a given rock based on data on melt inclusions in the early liquidus phases (olivine and pyroxene). In particular, we have demonstrated that the ore-bearing massifs were produced by highly magnesian (up to 8 wt % MgO) melts that contained olivine and plagioclase phenocrysts and had crustal characteristics: negative Ta-Nb and positive Pb anomalies and did not contain elevated concentrations of base metals. The melt contained 0.5-0.7 wt % H2O with low concentrations of Cl (0.2 wt %) and CO2 and its characteristics were close to those of lower crustal rocks (εNd = 0 ± 1.5; 87Sr/86Sr = 0.706 ± 0.1) that are are reasonable suitable candidates for the source of the Norilsk parental magmas. The possibility of melting is uncertain.
The two-stage scenario for the genesis of the sulfides seems to offer a more efficient mechanism for metal concentrating than a single-stage process.
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